The Trump administration on Monday said it is blocking American citizens in the Democratic Republic of Congo from traveling to the US on commercial flights, according to a White House official.

The order, which is being taken under a transportation authority known as Title 49, will place US citizens in Congo or those who have recently left on a “do-not-board” list until they have spent at least 21 days in a third country, the person said.

The new restrictions come amid a widening Ebola outbreak, which has spread to a number of provinces within Congo. The number of confirmed Ebola cases across the country had risen to 1 926, including 702 deaths, official data showed late on Sunday.

The often fatal viral disease spreads through direct contact with bodily fluids from infected people or animals and causes symptoms that can include high fever, vomiting and internal ⁠and external bleeding.

Some two dozen Americans were set to board flights to the US on Tuesday after having traveled to Congo, according to the US official, who said the State Department would support them and others affected during the waiting period.

Earlier on Monday, US Secretary of Health and Human Services Robert F ‌Kennedy Jr. signed an order citing increased Ebola risks, including the spread of the virus to just hours outside of Congo’s capital, Kinshasa.

The US Centers for Disease Control and Prevention (CDC) said on Friday that a US citizen working for a humanitarian organization in Congo had tested positive for the Bundibugyo Ebola virus; one American infected in Congo was admitted to Frankfurt University Hospital in Germany early on Monday, officials said. Another American, identified by the Serge Christian mission organization as Dr Peter Stafford, had contracted Ebola and had been brought to Germany for treatment, the CDC said in May.